For most of the current contrast-enhanced MR angiography (CE-MRA) examinations in the healthcare industry, acquisition is optimized for image contrast enhancement by matching the contrast arrival timing with the acquisition of the central phase encoding steps (i.e., time-to-center, TTC). The total scan time is kept short within a breath-hold length to suppress bulk breathing motion. Typically, the CE-MRA data are acquired without ECG-gating in a single continuous delayed centric trajectory. For most purposes, this approach is satisfactory. However, in CE-MRA of the thorax, the cardiac chambers and ventricular outflow vessels can be delineated with a certain degree of blurring with non-gated acquisition. To address this limitation, CE-MRA can be acquired with electrocardiography (ECG) gating, whereby the segmented data acquisition is synchronized with the cardiac cycle.
Conventional CE-MRA sequences support ECG gating with rigid trigger segmentation. For every trigger pulse, the conventional gated CE-MRA acquires all phase encoding steps for a single value of the slice encoding gradient. The acquisition is then repeated in linear order for all slice encoding values. With a suitable trigger delay (TD), the center of k-space in the inner loop (i.e. phase-encoding) direction (ky=0) can be acquired outside of the systolic phase, where the cardiac motions are most prominent. With proper contrast injection timing, the arterial window can still be matched with the center of the k-space in the outer loop (i.e. slice encoding) direction. With this scheme, the total scan time corresponds to the average R-R interval multiplied by the total number of slice encoding steps.
The conventional gated CE-MRA has two major unfulfilled needs. First, due to the rigid segmentation structure, the scan is very inefficient. For example, a typical high-resolution non-gated CE-MRA protocol uses short TR times of 2.7 ms and less than 200 phase encode steps in ky direction. Hence, the data acquisition window during each heartbeat is much shorter than the average R-R interval, which reduces the efficiency of the acquisition. Furthermore, the conventional gated CE-MRA technique cannot reduce scan time by taking advantage of parallel acquisition techniques (e.g., iPAT) and partial Fourier in phase encoding direction; if either of these parameters is modified, the total scan time remains the same since conventional CE-MRA only a single complete inner loop is played out per heartbeat, regardless of its duration. Secondly, the unpredictable nature of the in-vivo ECG-triggering adds some uncertainty to the gated CE-MRA. While the sequence assumes a steady R-R interval and uses a fixed acquisition window, due to physiological irregularities (e.g., the R-R interval can vary during a breath-hold) and mechanical imperfections (e.g., ECG detection device can fail), trigger events can be either detected too early or too late to substantially increase the scan time. Moreover, the CE-MRA sequence has a strict timing requirement with the contrast arrival, and any deviation from this may result in a contrast washout with missed optimal timing.